Generally, there are four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HC), oxides of nitrogen (NOx) and particulate matter (PM).
Oxidation catalysts have been used to clean up the hydrocarbon (HC), carbon monoxide (CO) and also the soluble organic fraction (SOF), in the exhaust gas produced by fuel combustion in internal combustion engines of the compression ignition type (see Japan Patent Kokai No. 9-271674). Recently attention has also focused on treatment of the particulate matter (PM) produced by fuel combustion in internal combustion engines of the compression ignition type, and filters that can collect PM (diesel particulate filters (DPF). An oxidation catalyst has been placed in the upstream part of the DPF with the aim of improving the efficiency of treatment of PM in the DPF (see Japan Patent Kokai No. 2006-272064).
Heavier fuels contain a greater sulphur fraction. In compression type internal combustion engines that use diesel oil as fuel, oxides of sulphur (SOx) are evolved in the fuel combustion and catalytic process, and the activity of the oxidation catalyst has been depressed by the presence of SOx (sulphur poisoning). To counter this problem, oxidation catalysts having resistance to sulphur poisoning have been proposed, which have zeolite as a mixture containing specific proportions by weight of ZSM-5 and β-zeolite (see Japan Patent Kokai No. 2007-229679). In addition, to make the cleaning of exhaust gas more efficient, an NOx cleaning catalyst having a double layer structure has been proposed that comprises two different catalyst layers, namely an NOx oxidation catalyst layer and NOx selective reduction catalyst layer (see Japan Patent Kokai 2008-279352).
As emissions standards for permissible emission of pollutants in exhaust gases from vehicular engines become progressively tightened, a combination of engine management and multiple catalyst exhaust gas after-treatment systems are being proposed and developed to meet these emission standards. For exhaust systems containing a particulate filter, it is common for engine management to be used periodically (e.g. every 500 km) to increase the temperature in the filter in order to combust substantially all remaining soot held on the filter thereby to return the system to a base-line level. These engine managed soot combustion events are often called “filter regeneration”. While a primary focus of filter regeneration is to combust soot held on the filter, an unintended consequence is that one or more catalyst coatings present in the exhaust system, e.g. a filter coating on the filter itself (a so-called catalysed soot filter (CSF)), an oxidation catalyst (such as a diesel oxidation catalyst (DOC)) or a NOx adsorber catalyst (NAC) located upstream or downstream of the filter (e.g. a first DOC followed by a diesel particulate filter, followed in turn by a second DOC and finally a SCR catalyst), can be regularly exposed to high exhaust gas temperatures, depending on the level of engine management control in the system. Such conditions may also be experienced with unintended occasional engine upset modes or uncontrolled or poorly controlled regeneration events. However, some diesel engines, particularly heavy duty diesel engines operating at high load, may even expose catalysts to significant temperatures, e.g. >600° C. under normal operating conditions.
As vehicle manufacturers develop their engines and engine management systems for meeting the emission standards, the Applicant/Assignee is being asked by the vehicle manufacturers to propose catalytic components and combinations of catalytic components to assist in the goal of meeting the emission standards. Such components include DOCs for oxidising CO, HCs and optionally NO also; CSFs for oxidising CO, HCs, optionally for oxidising NO also, and for trapping particulate matter for subsequent combustion; NACs for oxidising CO and HC and for oxidising nitrogen monoxide (NO) and absorbing it from a lean exhaust gas and to desorb adsorbed NOx and for reducing it to N2 in a rich exhaust gas (see below); and selective catalytic reduction (SCR) catalysts for reducing NOx to N2 in the presence of a nitrogenous reductant, such as ammonia (see below).
In practice, catalyst compositions employed in DOCs and CSFs are quite similar. Generally, however, a principle difference between the use of a DOC and a CSF is the substrate monolith onto which the catalyst composition is coated: in the case of a DOC, the substrate monolith is typically a flow-through substrate monolith, comprising a metal or ceramic honeycomb monolith having an array of elongate channels extending therethrough, which channels are open at both ends; a CSF substrate monolith is a filtering monolith such as a wall-flow filter, e.g. a ceramic porous filter substrate comprising a plurality of inlet channels arranged in parallel with a plurality of outlet channels, wherein each inlet channel and each outlet channel is defined in part by a ceramic wall of porous structure, wherein each inlet channel is alternately separated from an outlet channel by a ceramic wall of porous structure and vice versa. In other words, the wall-flow filter is a honeycomb arrangement defining a plurality of first channels plugged at an upstream end and a plurality of second channels not plugged at the upstream end but plugged at a downstream end. Channels vertically and laterally adjacent to a first channel are plugged at a downstream end. When viewed from either end, the alternately plugged and open ends of the channels take on the appearance of a chessboard.
Quite complicated multiple layered catalyst arrangements such as DOCs and NACs can be coated on a flow-through substrate monolith. Although it is possible to coat a surface of a filter monolith, e.g. an inlet channel surface of a wall-flow filter, with more than one layer of catalyst composition, an issue with coating filtering monoliths is to avoid unnecessarily increasing back-pressure, when in use, by overloading the filter monolith with catalyst washcoat, thereby restricting the passage of gas therethrough. Hence, although coating a surface of a filter substrate monolith sequentially with one or more different catalyst layers is not impossible, it is more common for different catalyst compositions to be segregated either in zones, e.g. axially segregated front and rear half zones of a filter monolith, or else by coating an inlet channel of a wall-flow filter substrate monolith with a first catalyst composition and an outlet channel thereof with a second catalyst composition. However, in particular embodiments of the present invention, the filter inlet is coated with one or more layers, which layers may be the same or a different catalyst composition. It has also been proposed to coat a NAC composition on a filtering substrate monolith (see e.g. EP 0766993).
In exhaust systems comprising multiple catalyst components, each comprising a separate substrate monolith, typically, the SCR catalyst is located downstream of a DOC and/or a CSF and/or a NAC because it is known that by oxidising some nitrogen oxide (NO) in the exhaust gas to nitrogen dioxide (NO2) so that there is about a 1:1 ratio of NO:NO2 exiting the DOC and/or the CSF and/or the NAC, the downstream SCR reaction is promoted (see below). It is also well known from EP341832 (the so-called Continuously Regenerating Trap or CRT®) that NO2, generated by oxidising NO in exhaust gas to NO2, can be used to combust soot passively on a downstream filter. In exhaust system arrangements where the process of EP341832 is important, were the SCR catalyst to be located upstream of the filter, this would reduce or prevent the process of combusting trapped soot in NO2, because a majority of the NOx used for combusting the soot would likely be removed on the SCR catalyst.
However, a preferred system arrangement for light-duty diesel vehicles is a diesel oxidation catalyst (DOC) followed by a nitrogenous reductant injector, then a SCR catalyst and finally a catalysed soot filter (CSF). A short hand for such an arrangement is “DOC/SCR/CSF”. This arrangement is preferred for light-duty diesel vehicles because an important consideration is to achieve NOx conversion in an exhaust system as quickly as is possible after a vehicle engine is started to enable (i) precursors of nitrogenous reductants such as ammonia to be injected/decomposed in order to liberate ammonia for NOx conversion; and (ii) as high NOx conversion as possible. Were a large thermal mass filter to be placed upstream of the SCR catalyst, i.e. between the DOC and the SCR catalyst (“DOC/CSF/SCR”), the processes of (i) and (ii) would take far longer to achieve and NOx conversion as a whole of the emission standard drive cycle could be reduced. Particulate removal can be done using oxygen and occasional forced regeneration of the filter using engine management techniques.
It has also been proposed to coat a SCR catalyst washcoat on a filter substrate monolith itself (see e.g. WO 2005/016497), in which case an oxidation catalyst may be located upstream of the SCR-coated filter substrate (whether the oxidation catalyst is a component of a DOC, a CSF or a NAC) in order to modify the NO/NO2 ratio for promoting NOx reduction activity on the SCR catalyst. There have also been proposals to locate a NAC upstream of a SCR catalyst disposed on a flow-through substrate monolith, which NAC can generate NH3 in situ during regeneration of the NAC (see below). One such proposal is disclosed in GB 2375059.
NACs are known e.g. from U.S. Pat. No. 5,473,887 and are designed to adsorb NOx from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is decreased. Desorbed NOx may be reduced to N2 with a suitable reductant, e.g. engine fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, control of oxygen concentration can be adjusted to a desired redox composition intermittently in response to a calculated remaining NOx adsorption capacity of the NAC, e.g. richer than normal engine running operation (but still lean of stoichiometric or lambda=1 composition), stoichiometric or rich of stoichiometric (lambda<1). The oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold.
A typical NAC formulation includes a catalytic oxidation component, such as platinum, a significant quantity (i.e. substantially more than is required for use as a promoter such as a promoter in a three-way catalyst) of a NOx-storage component, such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:NO+½O2→NO2  (1); andBaO+2NO2+½O2→Ba(NO3)2  (2),wherein in reaction (1), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (2) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).Ba(NO3)2→BaO+2NO+3/2 O2 or Ba(NO3)2→BaO+2NO2+½O2  (3); andNO+CO→½N2+CO2  (4);(Other reactions include Ba(NO3)2+8H2→BaO+2NH3+5H2O followed by NH3+NOx→N2+yH2O or 2NH3+2O2+CO→N2+3H2O+CO2 etc.).
In the reactions of (1)-(4) inclusive herein above, the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide and sequence of catalytic coatings in the exhaust stream.
Oxidation catalysts promote the oxidation of CO to CO2 and unburned HCs to CO2 and H2O. Typical oxidation catalysts include platinum and/or palladium on a high surface area support.
The application of SCR technology to treat NOx emissions from vehicular internal combustion (IC) engines, particularly lean-burn IC engines, is well known. Examples of nitrogenous reductants that may be used in the SCR reaction include compounds such as nitrogen hydrides, e.g. ammonia (NH3) or hydrazine, or an NH3 precursor.
NH3 precursors are one or more compounds from which NH3 can be derived, e.g. by hydrolysis. Decomposition of the precursor to ammonia and other by-products can be by hydrothermal or catalytic hydrolysis. NH3 precursors include urea (CO(NH2)2) as an aqueous solution or as a solid or ammonium carbamate (NH2COONH4). If the urea is used as an aqueous solution, a eutectic mixture, e.g. a 32.5% NH3 (aq), is preferred. Additives can be included in the aqueous solutions to reduce the crystallisation temperature. Presently, urea is the preferred source of NH3 for mobile applications because it is less toxic than NH3, it is easy to transport and handle, is inexpensive and commonly available. Incomplete hydrolysis of urea can lead to increased PM emissions on tests for meeting the relevant emission test cycle because partially hydrolysed urea solids or droplets will be trapped by the filter paper used in the legislative test for PM and counted as PM mass. Furthermore, the release of certain products of incomplete urea hydrolysis, such as cyanuric acid, is environmentally undesirable.
SCR has three main reactions (represented below in reactions (5)-(7) inclusive) which reduce NOx to elemental nitrogen.4NH3+4NO+O2→4N2+6H2O (i.e. 1:1 NH3:NO)  (5)4NH3+2NO+2NO2→4N2+6H2O (i.e. 1:1 NH3:NOx)  (6)8NH3+6NO2→7N2+12H2O (i.e. 4:3 NH3:NOx)  (7)
A relevant undesirable, non-selective side-reaction is:2NH3+2NO2→N2O+3H2O+N2  (8)
In practice, reaction (7) is relatively slow compared with reaction (5) and reaction (6) is quickest of all. For this reason, when skilled technologists design exhaust aftertreatment systems for vehicles, they often prefer to dispose an oxidation catalyst element (e.g. a DOC and/or a CSF and/or a NAC) upstream of an SCR catalyst.
It has been brought to the Applicant/Assignee's attention by its customers that when certain DOCs and/or NACs and/or CSFs become exposed to the high temperatures encountered e.g. during filter regeneration and/or an engine upset event and/or (in certain heavy-duty diesel applications) normal high temperature exhaust gas, it is possible given sufficient time at high temperature for low levels of platinum group metal components, particularly Pt, to volatilise from the DOC and/or the NAC and/or the CSF components and subsequently for the platinum group metal to become trapped on a downstream SCR catalyst. This can have a highly detrimental effect on the performance of the SCR catalyst, since the presence of Pt leads to a high activity for competing, non-selective ammonia oxidation such as in reaction (9) (which shows the complete oxidation of NH3), thereby producing secondary emissions and/or unproductively consuming NH3.4NH3+5O2→4NO+6H2O  (9)
One vehicle manufacturer has reported the observation of this phenomenon in SAE paper 2009-01-0627, which is entitled “Impact and Prevention of Ultra-Low Contamination of Platinum Group Metals on SCR catalysts Due to DOC Design” and includes data comparing the NOx conversion activity against temperature for a Fe/zeolite SCR catalyst located in series behind four suppliers' platinum group metal (PGM)-containing DOCs that were contacted with a flowing model exhaust gas at 850° C. for 16 hours. The results presented show that the NOx conversion activity of a Fe/zeolite SCR catalyst disposed behind a 20Pt:Pd DOC at 70gft−3 total PGM was negatively altered at higher evaluation temperatures as compared to lower evaluation temperatures as a result of Pt contamination. Two 2 Pt:Pd DOCs from different suppliers at 105 gft−3 total PGM were also tested. In a first 2Pt:Pd DOC, the SCR catalyst activity was affected to a similar extent as the test on the 20Pt:Pd DOC, whereas for the second 2Pt:Pd DOC tested the SCR catalyst activity was contaminated to a lesser extent, although the second 2Pt:Pd DOC still showed reduced NOx conversion activity compared with the blank control (no DOC, just a bare substrate). The authors concluded that the supplier of the second 2Pt:Pd DOC, which showed more moderate NOx conversion degradation, was more successful in stabilising the 70gft−3 Pt present with the 35gft−3 Pd. A Pd-only DOC at 150 gft−3 demonstrated no impact on the downstream SCR relative to the blank control. Earlier work from the authors of SAE 2009-01-0627 was published in SAE paper no. 2008-01-2488.
Vehicle manufacturers have begun asking the Applicant/Assignee for measures to solve the problem of volatilisation of relatively low levels PGMs from components upstream of SCR catalysts. It would be highly desirable to develop strategies to prevent this PGM movement onto a downstream SCR catalyst at high temperatures. The present inventors have developed a number of strategies for meeting this need.
U.S. Pat. No. 7,576,031 discloses a Pt-Pd diesel oxidation catalyst with CO/HC light-off and HC storage function. In particular, the diesel oxidation catalyst comprises a washcoat composition comprising two distinct washcoat layers. A first (or top) washcoat layer comprises a high-surface area support material, one or more hydrocarbon storage components, and a precious metal catalyst containing platinum (Pt) and palladium (Pd). The second (or bottom) washcoat layer comprises a high-surface area support material and a precious metal catalyst containing platinum (Pt) and palladium (Pd), wherein the support is a substantially silica free support material and does not contain a hydrocarbon storage component.
The two layers of the diesel oxidation catalyst disclosed in U.S. Pat. No. 7,576,031 have two distinctly different weight ratios of Pt:Pd relative to one another, wherein the Pt:Pd weight ratio in a first layer (the first or top washcoat layer) is greater than the Pt:Pd weight ratio of a second layer (the second or bottom washcoat layer). For example, the first or top washcoat layer may contain a Pt:Pd weight ratio of at least 2:1. Pt:Pd weight ratios from at least about 2:1 to about 10:1, from about 3:1 to about 5:1, or from about 3:1 to about 4:1, are also exemplified. It is explained that it is important to use a high amount of Pt in the first or top washcoat layer in order to boost sulfur tolerance while maintaining some stabilization of the metal phase against sintering. The first or top washcoat layer contains a hydrocarbon (HC) storage component, e.g., a zeolite, in order to store HCs during the cold start period of the drive cycle. After warm-up of the catalyst, the hydrocarbon (HC) storage component will release the stored HCs which are subsequently converted over the catalyst. It is important, the description continues, that the hydrocarbon (HC) storage component (e.g., zeolite) be incorporated into the layer with the higher Pt:Pd weight ratio in order to ensure an efficient conversion of released paraffins.
The second or bottom layer of the diesel oxidation catalyst disclosed in U.S. Pat. No. 7,576,031 contains a lower Pt:Pd weight ratio to replace a maximum of the Pt with Pd for maximum cost saving reasons. The second or bottom washcoat layer has a Pt:Pd weight ratio of less than about 2:1. Also, exemplified are Pt:Pd ratios of from less than about 2:1 to about 1:2, or from less than about 2:1 to about 1.4:1 (7:5). However, a minimum ratio of 1.4:1 (7:5) is preferred in order to guarantee sufficient CO/olefin light-off activity after thermal aging.